1. Field of the Invention
This invention relates to an ion-implantation method for implanting impurities in solid materials, and more particularly, to an ion-implantation method for implanting ionized impurities in semiconductor substrates that is suitable for the manufacture of insulated-gate field-effect semiconductor devices.
2. Description of the Prior Art
With the miniaturization of semiconductor devices such as integrated circuits, an ion-implantation method by which atom impurities are introduced uniformly into the semiconductor crystals is a very important process during the manufacture of, for example, VLSI. In particular, this method is essential to the step in which a gate oxide film is formed to have a side wall so that a lightly doped drain (LDD) structure is constructed, and the method is widely used.
FIG. 3 shows fragmentary sectional views of a MOS transistor with an ordinary LDD structure. The MOS transistor is produced as follows: (a) first, by use of an ordinary method for the manufacture of MOS transistors, a gate oxide film 42 is formed by thermal oxidation on a P-type silicon substrate 41, and a gate electrode film 43 is formed on the gate oxide film 42. Next, a photoresist layer is formed over the entire surface, and by photolithography, a photoresist pattern 44 for use in the formation of a gate electrode is formed as shown in FIG. 3a; (b) the photoresist pattern 44 is used as a mask, and the gate electrode film 43 and the gate oxide film 42 are etched in this order, (c) after the photoresist pattern 44 has been removed, the resulting gate electrode 45 and the gate oxide film 42 are used as a shielding mask, .sup.31 P.sup.+ ions (shown by the ".DELTA." marks in FIG. 3c) are implanted in the P-type silicon substrate 41, resulting in a low-concentration region of the LDD structure; (d) an insulating film 46 is formed over the entire surface; (e) next, anisotropic etching is conducted of the insulating film 46, and a blocking film is formed to have a side wall 47 on the gate electrode 45; (f) in this situation, .sup.75 As.sup.+ ions (shown by the "x" marks in FIG. 3f) are implanted in the P-type silicon substrate 41, resulting in a high-concentration region of the LDD structure; (g) thereafter, by annealing of the ion-implanted layer, a low-concentration region of impurities (n.sup.- -part 48) and a high-concentration region of impurities (n.sup.+ -part 49) are formed, which results in a device with an LDD structure.
However, when the regions with impurities are formed by the conventional ion-implantation method of this type, as shown in FIG. 3h, crystal defects 50 and 51 are concentrated under the edge of the blocking film that is used as a shielding mask for ion implantation. In fact, vertical sections of the substrate were observed by transmission electron microscopy, and it was found that when crystal defects occur deep in the substrate, the crystal defects cannot be removed by a later step of annealing. Therefore, leakage current and other problems can arise, and cause difficulties in the functioning of the device.
The causes of the concentration of crystal defects under the edge of the blocking film as described above are not clearly known yet, but the main causes may include the following:
One is that because the blocking film is formed on a side wall of the gate electrode for an LDD structure, the stress of this blocking film is concentrated on the edges of the source side and the drain side, which may give rise to crystal defects.
Another possibility is that as shown in FIG. 3h, the crystal defects 50 and 51, which are caused by ion-implantation damage, are mixed with each other at the edges of the source side and the drain side. As shown in FIG. 4a, when ion implantation is conducted with an oxide film (side wall 47) used as a shielding mask, the distribution of the implanted impurities can be adequately approximated by the Gaussian distribution. The line A-A' in FIG. 4a shows a selected ordinary ion-implanted layer; a model of the impurity profile on the vertical section at different depths is shown in FIG. 4b.
Next, line B-B' shows a selected ion-implanted layer just below the edge of the blocking film for ion implantation, and a model of the impurity profile on the vertical section at different depths is shown in FIG. 4c.
In the same way, line C-C' shows a selected ion-implanted layer that is further inside the blocking film for ion implantation than line B-B'; a model of the impurity profile on the vertical section at depths is shown in FIG. 4d.
As shown in FIG. 4b, the impurity profile on the vertical section taken along line A-A' of FIG. 4a can be adequately approximated by a model in which the concentration distribution of implanted impurities has a peak at several hundreds of angstroms or several thousands of angstroms from the surface of the semiconductor substrate and a gentle gradient toward the inside of the substrate. The impurity concentration profile on the vertical section taken along line A-A' shows that there are impurities implanted in the semiconductor substrate with the distribution shown in FIG. 4b.
With ion implantation, the accelerated ions collide with the atoms and electrons of the semiconductor crystal, and lose their energy, so that they become stationary in the semiconductor crystal. Therefore, the semiconductor crystal has the most irregularities of its crystal structure in the region around the crystal defects I of FIG. 4b. Moreover, there are large numbers of impurities in this region that cannot make a solid solution, and this may give rise to crystal defects that are very difficult to remove by later heat treatment.
The crystal defects II in FIG. 4b are near the interface between the crystalline region and the amorphous region of the semiconductor substrate. At this area, the distortion and/or lattice mismatching in the semiconductor substrate, which is caused by ion-implantation damage, have become a nucleus, and during a later step of annealing, this nucleus may give rise to large crystal defects such as dislocations and the like. When a high concentration of ions (10.sup.15 /cm.sup.2 or more) is implanted in the semiconductor substrate so that a source region and a drain region can be formed, there can exist together both point defects, which do not constitute a continuous amorphous region, and island-like amorphous regions in the areas near the crystal defects II. Also, there seem to be crystal defects that are usually difficult to remove by later heat treatment.
The inventors of this invention found the occurrence of these kinds of crystal defects through observations by transmission electron microscopy. These crystal defects appear, as shown in FIG. 3h, at a position 50 (crystal defects I) reached by the implanted impurities and also at a position 51 (crystal defects II) that is about twice as deep.
In FIG. 4c and 4d, as explained above in detail, in order to discuss the distribution of the implanted impurities under the edge of the blocking film, the impurity profile on the vertical section taken along B-B' of FIG. 4a and the impurity profile on the vertical section taken along line C-C' of FIG. 4a are expressed in terms of a very simple model (in practice, it is necessary to consider a number of factors, such as the shape of the ion beam, the density of the ion beam, the dose-rate effect, and the radiation enhancement diffusion (RED), but these factors are not considered herein). As can be seen from FIG. 4c, in the distribution under the edge of the blocking film (the impurity profile on the vertical section taken along line B-B'), crystal defects I and II are closer to each other than in the usual distribution of FIG. 4b (the impurity profile on the vertical section taken along line A-A'). Also, in FIG. 4d (the impurity profile on the vertical section taken along line C-C'), the crystal defects I and II can be seen to be close enough to overlap with each other. This suggests that the crystal defects that are caused by ion-implantation damage will be mixed with each other inside the semiconductor substrate crystal at the area just below the edges of the source side and the drain side of the blocking film.
The two points that are explained above can be thought of as being the main causes of the occurrence of crystal defects in the inside of the semiconductor substrate under the edge of the blocking film for ion implantation, the crystal defects that already occurred not being removed by later heat treatment. It should be noted that in the case of a semiconductor substrate in which no impurities are implanted, there are no crystal defects in the areas under the edge of the side wall of the blocking film. Taking this fact into consideration, it seems that crystal defects occur in the areas under the edge of the blocking film because of damage during a step of ion implantation or as a synergistic effect between the ion-implantation damage and the stress generated in the side wall of the blocking film.